Gas turbine engines, such as those found on jet aircraft, comprise a housing within which fuel and air are combined and burned. The resulting hot exhaust gasses are used to turn a turbine and provide thrust when they exit the housing. Such engines generally include a fan to draw air into the housing and a compressor that compresses the air and sends it to a gas generator. Here a precisely metered supply of fuel is mixed with the compressed air and burned. The expanding exhaust gasses turn a turbine which powers at least the compressor and the fan. The exhaust gasses then pass through the remainder of the housing and exit the housing to provide thrust.
Such engines sometimes include sections called “afterburners” or “thrust augmentors” or merely “augmentors” that allow a gas turbine engine to produce additional thrust for limited periods of time—to help an aircraft take-off, or during critical military maneuvers, for example. Augmentors may comprise a set of fuel nozzles located in a chamber downstream from the gas generator in which chamber additional fuel is burned to increase engine thrust. Augmentor nozzles may receive fuel over a different supply path than the supply path used to provide fuel to the gas generator. Thus, the use of the augmentor does not reduce fuel supplied to the gas generator.
The supply of fuel to the gas generator must be metered very precisely and maintained substantially continuously to ensure reliable engine operation. Therefore, on many gas turbine engines it is desirable to have a backup system to provide a metered flow of fuel to the gas generator to allow continued engine operation in the event of a primary fuel metering system failure. Typically these backup systems are implemented by added redundant parts and control transfer systems.
For some critical gas turbine engine operating conditions, the gas generator fuel flow metering system should be able to accommodate a failure with minimal impact to the metered flow. In some aircraft systems, it has been determined that a reduction in fuel flow of 20% for more than 70 ms will result in loss of the aircraft. Achieving this level of failure accommodation with an active/standby system is extremely difficult. Even typical active/active approaches, using two servo valves to control one metering, can not achieve this fault accommodation without extreme measures that add a substantial weight and cost to the system.